Silver crystals: These octahedral silver nanoparticles suspended in ethanol are assembling into a large crystal. The density of the particles, which changes from top to bottom, determines what colors pass through.

Results: Researchers at the University of California, Berkeley, led by Peidong Yang, have shown that silver nanoparticles with very regular octahedral shapes pack together under the influence of gravity to form large crystals. The crystals’ optical properties can be varied by changing the amount of time the nanoparticles have to pack together, which affects the spacing between them.

Why it matters: Light striking the crystals causes the formation of what’s called a plasmon, a wave passing through the electrons at the crystals’ surfaces. Plasmonic crystals could be used to guide light in optical computers or to increase the sensitivity of chemical sensors. They could also serve as lenses for superhigh-resolution microscopy. Using conventional lithography to etch patterns in materials can achieve a similar effect but is more expensive.

Method: Using methods that Yang developed previously, the researchers grew silver nanoparticles in solution, then suspended them in ethanol inside a test tube. By allowing the nanoparticles to pack together for longer or shorter periods of time before evaporating the ethanol, the researchers produced densely and loosely packed crystals, whose optical properties they studied. The crystals transmitted particular bands of radiation while blocking others, and the frequency varied according to how tightly packed the crystals were.

Next steps: The Berkeley group plans to build a large plasmonic crystal on the surface of a six-inch wafer to establish that the crystals can be formed on a scale large enough for many of their potential applications.